The present invention relates to laser micromachining and, in particular, to a method and apparatus employing a single pass actuation (SPA) assembly to vary the power density of ultraviolet laser output applied to a target surface during processing of multilayer workpieces having at least two layers with different absorption characteristics in response to ultraviolet light.
The background is presented herein only by way of example to multilayer electronic workpieces, such as integrated-circuit chip packages, multichip modules (MCMs) and high-density interconnect circuit boards, that have become the most preferred components of the electronics packaging industry.
Devices for packaging single chips such as ball grid arrays, pin grid arrays, circuit boards, and hybrid microcircuits typically include separate component layers of metal and an organic dielectric and/or reinforcement materials, as well as other new materials. A standard metal component layer typically has a depth or thickness of greater than 5 xcexcm, a standard organic dielectric layer typically has a thickness of greater than 30 xcexcm, and a standard reinforcement component xe2x80x9clayerxe2x80x9d typically has a thickness of greater than 5 xcexcm disbursed throughout the dielectric layer. Stacks having several layers of metal, dielectric, and reinforcement material are often thicker than 2 mm.
Much recent work has been directed toward developing laser-based micromachining techniques to form vias in, or otherwise process, these types of electronic materials. Vias are discussed herein only by way of example to micromachining and may take the form of complete through-holes or incomplete holes called blind vias. Unfortunately, laser micromachining encompasses numerous variables including laser types, operating costs, and laser- and target material-specific operating parameters such as beam wavelength, power, and spot size, such that the resulting machining throughputs and hole quality vary widely.
In U.S. Pat. No. 5,593,606, Owen et al. describe advantages of employing UV laser systems to generate laser output pulses within advantageous parameters to form vias through at least two layers of multilayer devices. These parameters generally include nonexcimer output pulses having temporal pulse widths of shorter than 100 ns, spot areas with spot diameters of less than 100 xcexcm, and average intensities or irradiances of greater than 100 mW over the spot areas at repetition rates of greater than 200 Hz.
In U.S. Pat. No. 5,841,099, Owen et al. vary laser output within similar parameters to those described above to have different power densities while machining different materials. They change the intensity by changing the laser repetition rate and/or the spot size. In one embodiment, they employ a first laser output of a high intensity to ablate a metallic layer and a second laser output of lower intensity to ablate an underlying dielectric layer so a lower metal layer can act as a laser etch stop in blind via operations.
In one implementation, Owen et al. change spot size by raising and lowering the objective lens to change the energy density of the laser spot impinging upon the, workpiece. In most conventional laser systems, changing the height of the objective lens is a slow process because moving the vertical (Z) stage requires at least several hundred milliseconds (ms).
In another implementation, Owen et al. change the repetition rate of the laser to change the energy density of the laser spot impinging the workpiece. However, for a given laser power, if the energy per pulse is decreased, for example, by increasing the repetition rate, then more pulses and consequently more time is needed to apply the total energy that must be delivered to the workpiece to drill the via. Thus, this implementation also generally impacts throughput.
Even within the parameters established by Owen et al., skilled persons would need to further tailor the repetition rate changes and other process parameters to suit particular workpieces to produce vias meeting all the criteria for quality, including the via wall taper, the degree of melting of the copper layer at the bottom of the via, and the height of the xe2x80x9crimxe2x80x9d around the periphery of the via caused by the splash of molten copper during drilling. These parameters are difficult to optimize for throughput as well as for all the criteria for quality.
Because these energy density changing methods are time consuming or complex, the conventional process for machining through multilayer devices is typically at least a two pass operation. Such two pass operations involve sequentially removing a first layer of a first material at all of the desired target locations at a first energy density. Once all of the holes are made through the first layer, the spot size and/or repetition rate is changed to achieve a second energy density, which is then used to remove a second layer of a second material at all of the desired target locations.
The major disadvantage of such two pass operations is that the typical hole-to hole move time of 2-10 ms is relatively slow and each hole must be addressed twice, resulting in a total via formation time of 4-20 ms plus actual drilling time. FIG. 1 shows a best-case conventional time line for a double-pass, two-step via drilling process, assuming a hole drilling time of 2 ms and a 2 ms move time.
A faster and more reliable way of changing the energy density of laser output between first and second layer machining operations is therefore desirable.
An object of the present invention is, therefore, to provide a method or apparatus for quickly changing the energy density of laser output to facilitate machining of workpieces.
Another object of the invention is to improve the throughput of workpieces in such laser machining operations.
A further object of the invention is to facilitate one-pass processing of workpieces.
Changing the laser spot size is more practical than changing the repetition rate to alter the energy density because if a laser system decreases the power density by maintaining the energy per pulse but spreading it out over a larger laser spot area, the laser system can apply the same total energy with fewer pulses. Hence, the system can process the workpieces faster. The present invention preferably, therefore, conserves the total energy per pulse and employs a system or method that rapidly changes the area of the laser spot impinging upon the workpiece. By changing the laser spot size in a period of less than a few milliseconds, the present invention can eliminate the conventional second pass of hole-to-hole moves, and the throughput of the overall process can be substantially increased.
The present invention employs a single pass actuation assembly to change the energy density of laser output pulses between at least two different values to facilitate processing different layers at different energy densities. In a preferred embodiment of the present invention, a deformable mirror permits a quick, preferably less than one millisecond, change of focus of UV laser output to change the spot size of the focused beam waist and hence its energy density without requiring Z-axis movement of the laser positioning system.
Deformable mirrors have been employed as adaptive optics for IR- and visible-wavelength lasers in astronomy and climatology applications to compensate for atmospheric turbulence in order to keep the fluence constant.
In U.S. Pat. No. 5,667,707, Klingel et al. employ a laser system with a deformable mirror to cut or weld metal of huge panels having surfaces that are not particularly flat. Their laser operation requires a high-energy tightly-focused laser spot to efficiently process the metal target. They employ the deformable mirror to change the focus height of the laser spot to maintain the size of the laser spot at the target surface regardless of its flatness and hence maintain the laser spot""s high fluence throughout the metal processing operation. The deformable mirror has a soft surface whose curvature is manipulated by varying fluid pressure. The mirror response time is relatively slow.
A preferred deformable mirror employs a flexible face sheet made from an optically flat and coated machined piece of glass or other common optical substrate that is rigidly attached to two concentric circles of an electrostrictive actuator, preferably made of PMN (lead magnesium niobate). The outer circle of the actuator is active and increases in length with applied voltage. The inner circle is not connected to power and is therefore inactive. Whenever a voltage is applied to the actuator, the outer PMN material expands, pushing on the outer rim of the face sheet while the inner PMN material holds the center of the face sheet firmly in position. The resulting surface contour of the face sheet is concave. The back of the face sheet is machined such that the active concave surface contour is smooth and continuous and has the correct optical figure over its clear aperture so the reflected beam wavefront is precisely spherical. The use of the PMN electrostrictive actuator allows focus changes to be accomplished in less than about 0.5 ms.
Other embodiments may employ galvanometer-driven mirrors to divert the laser beam to an alternative focal path to change the size of the laser spot area.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.